Some condenser microphones have an interchangeable head unit (or head section), which includes a condenser microphone unit as an electrostatic electroacoustic transducer, on the microphone body (or grip section for a hand-held microphone) for the purpose of changing the directivity.
In such type of microphone, however, an electrical connection is present between the head unit and the microphone body, presenting an extremely high impedance at a signal output part of the condenser microphone unit. Consequently, noises are likely to be generated when external electromagnetic waves radiated from, for example, a mobile phone are arriving from the outside.
Thus, the applicant has proposed in Japanese Patent Application Publication No. 2006-313952 to connect an inductor used as a choke coil in series between a ground on the head unit side and another ground on the microphone body side.
As a matter of fact, however, noises have not yet been suppressed low enough to reach a satisfactory level according to Japanese Patent Application Publication No. 2006-313952 due to low signal levels and the extremely high impedance of the condenser microphone.
Generally, a field-effect transistor (FET) is used for an impedance converter for a condenser microphone. There are two ways to extract signals from a condenser microphone having an FET on the head unit side: to extract sound signals from the drain side of the FET and to extract sound signals from the source side of the FET.
In the case of extracting sound signals from the drain side of the FET, the drain line serves as both a feed line and a signal output line. Thus, since only one connection can be provided between the head unit and the microphone body, the structure can be simplified. Unfortunately, sound signals are easily distorted due to a low level of the signals.
On the other hand, in the case of extracting sound signals from the source side of the FET, sound signals are not distorted to a large extent. In this case, however, two connections are required because the power must be separated from the signals, resulting in a complex structure.
In the meantime, to design a condenser microphone operated with a phantom power supply, it is necessary to take into consideration a feed voltage from the phantom power supply.
According to EIAJ RC-8162A “Power Supply Method for Microphone,” 3 allowable rated voltages are defined for phantom power supplies for condenser microphone: 12±1V, 24±4V, and 48±4V.
In order to operate a microphone with a higher maximum output level within the respective voltage range of phantom power supplies, which have such different rated voltages, the applicant has proposed Japanese Patent Application Publication No. 2006-352622 and Japanese Patent Application Publication No. 2007-006178.
An object of the present invention is to provide a condenser microphone operated with a phantom power supply, the condenser microphone having an interchangeable head unit on the microphone body, wherein the head unit can be attached and detached to and from the microphone body through one connection at a low impedance.